Coated sinter of cubic-system boron nitride

ABSTRACT

A coated cubic boron nitride (cBN) sintered body most suitable for a cutting tool having excellent resistance to wear and heat in the high-speed cutting of steel has been developed. The sintered body comprises (a) a base material made of a sintered body comprising at least 99.5 vol. % cBN and (b) a hard coating 0.1 to 10 μm in thickness formed on at least part of the surface of the base material by the PVD method. It is desirable that the hard coating comprise at least one compound layer consisting mainly of (a) at least one metal element selected from Al and the IV a-group elements and (b) at least one element selected from C, N, and O. It is desirable to provide between the base material and the hard coating an intermediate layer comprising a compound consisting mainly of boron and at least one metal element selected from the IV a-group elements.

TECHNICAL FIELD

[0001] The present invention relates to a coated cubic boron nitridesintered body in which a sintered body consisting mainly of cubic boronnitride (hereinafter referred to as cBN) is coated with a hard layer,particularly a coated cBN sintered body that is most suitable for acutting tool having excellent resistance to wear and heat in thehigh-speed cutting of steel.

BACKGROUND ART

[0002] The hardest material next to diamond, cBN has been used in a cBNsintered body in which cBN is sintered at superhigh pressures togetherwith a 10 to 60 vol. % binder, such as TiN, TiC, Al, or Co. These cBNsintered bodies have been available in the market mainly for use intools for the cutting of hardened steel and cast iron.

[0003] There is another type of cBN sintered body, which is producedwithout using a binder. The sintered body is produced byreaction-sintering hexagonal boron nitride (hBN) with the assistance ofa catalyst such as magnesium boronitride. The sintered body has athermal conductivity as high as 600 to 700 W/m·K, enabling its use as aheatsink material and a TAB bonding tool. The sintered body, however,has some amount of residual catalyst. When heated, the sintered bodytends to form ninute cracks because its thermal expansion differs fromthat of the residual catalyst. As a result, its maximum allowabletemperature is as low as about 700° C. In addition, because the cBN hasa crystal-grain diameter as large as about 10 μm, the cBN sintered bodyis insufficient in strength, despite its high thermal conductivity. Thislow mechanical strength has precluded its use as a cutting tool.

[0004] On the other hand, cBN can also be synthesized by treatinglow-pressure BN, such as hBN, at superhigh pressures and hightemperatures without using a catalyst. This process is called directconversion. It is known that concurrent sintering with this hBN/cBNconversion can produce a cBN sintered body containing no binder. Forexample, the published Japanese patent applications Tokukaishou 47-34099and Tokukaihei 3-159964 have disclosed a method of producing a cBNsintered body by converting hBN into cBN at superhigh pressures and hightemperatures. The other applications Tokukoushou 63-394 and Tokukaihei8-47801 have disclosed another method of producing a cBN sintered bodyby using pyrolytic boron nitride (pBN) as the material, proposing theuse of the sintered body for the high-speed cutting of gray cast ironthat has relatively good machinability.

[0005] However, recent years have seen an advancement in the trendtoward the high-speed cutting of steel in order to improve machiningefficiency and toward high-speed cutting under dry conditions in orderto reduce the adverse effects on the environment. Under suchcircumstances, conventional tools, such as a tool made of a coatedcemented carbide, have been disadvantageous because they cause problems,such as (a) a reduced tool-life due to wear resulting from the increasein the temperature and load of the cutting edge while the tool iscutting, (b) plastic deformation of the base material, (c) crackformation due to heat shock, and (d) damage resulting from insufficientstrength at high temperatures.

[0006] In order to solve these problems, another application,Tokukaishou 59-8679, has proposed a method of coating a cBN sinteredbody having high hardness at high temperatures with a layer of Al₂O₃ ora composite layer of Al₂O₃ and any of TIC, TiN, and TiB. Yet anotherapplication, Tokukaishou 61-183187, has disclosed a method of coatingthe cBN sintered body stated in Tokukaishou 59-8679 with TiN, TiC, orTiCN by the physical vapor deposition (PVD) method to improve wearresistance in the cutting of cast iron and steel.

[0007] However, these coated sintered bodies are produced by using asthe base material a cBN sintered body having insufficient heatresistance and strength at high temperatures. Consequently, when theyare used for the high-speed cutting of steel, the coating spalls awaydue to the temperature rise during the cutting, resulting in theaccelerated progression of wear. In addition, the base material of thesecoated sintered bodies has low thermal conductivity and a largecoefficient of thermal expansion. Therefore, when they are usedparticularly for intermittent high-speed cutting, thermal cracks areformed in the base material due to intense heat shock, without showingnotable improvement in resistance to wear and chipping.

[0008] A cBN sintered body containing no binder shows excellentresistance to heat and wear when used for the high-speed cutting of castiron. However, when used for the cutting of steel, the sintered bodyshows accelerated progression in the wear of the cutting edge due to thereaction between the cBN and steel, significantly reducing the toollife.

[0009] Considering the above-described circumstances, an object of thepresent invention is to offer a coated cBN sintered body that is mostsuitable for a cutting tool having excellent resistance to wear and heatparticularly in the high-speed cutting of steel.

[0010] Another object of the present invention is to offer a coated cBNsintered body for long-life cutting tools by firmly bonding a hardcoating having excellent resistance to wear and heat to the surface ofthe base material made of a cBN sintered body having excellent crackresistance at high temperatures.

DISCLOSURE OF THE INVENTION

[0011] The present inventors studied the heat resistance and heat-shockresistance required for a cBN sintered body to be used as a basematerial, and found that when conventional cBN sintered bodies are usedparticularly for the cutting of low-hardness steel, the reaction betweenthe cBN particles and the ferrous material accelerates the wear, causingthe development of flank wear and face wear and the subsequent reductionin tool life due to wear or the development of chipping. The presentinventors also found that high-speed cutting raises the tooltemperature, accelerates the softening and deterioration of the binder,reduces the resistance to heat and heat shock of the sintered body, andthereby significantly shortens the tool life. Based on the abovefindings, the present invention achieves the above-described objects byproviding a hard coating capable of improving wear resistance on atleast part of the base material made of a sintered body comprisingalmost solely cBN and by specifying the thickness of the coating

[0012] More specifically, the coated cBN sintered body of the presentinvention comprises (a) a base material made of a sintered bodycomprising at least 99.5 vol % cBN and (b) a hard coating 0.1 to 10 μmin thickness formed on the surface of the base material by the PVDmethod In addition, the bonding quality of the hard coating can beimproved by providing a specific intermediate layer between the basematerial and the hard coating. The constituents of the present inventionare explained in detail below.

Base Material

[0013] The present invention features a base material containing atleast 99.5 vol. % cBN. If less than 99.5 vol. %, the strength at hightemperatures decreases due to the deterioration of the substancescoexisting with the cBN, such as impurities, because they usually havelow heat resistance in comparison with cBN.

[0014] When the content of cBN is at least 99.5 vol. %, the wearresistance of the tool can be significantly improved because thereduction in the hardness of the base material is small even at hightemperatures, which enables the hard coating to maintain its highhardness. It is particularly desirable that the content of cBN be atleast 99.9 vol. % to secure the intrisincally excellent heat resistanceof cBN.

[0015] It is desirable that a base material comprise cBN having anaverage crystal-grain diameter of at most 1 μm. Such a smallcrystal-grain diameter of the cBN enables further improvement of thestrength of the sintered body. If the average crystal-grain diameterexceeds 1 μm, the boundary areas of the cBN grains decrease, causing anacceleration of the propagation of cracks, and thus decreasing thestrength.

[0016] It is desirable that the ratio I₍₂₂₀₎/I₍₁₁₁₎ be at least 0.05 inthe X-ray diffraction lines in an arbitrary direction of a basematerial, where I₍₂₂₀₎ denotes the intensity of the X-ray diffraction atthe (220) plane of the cBN, and I₍₁₁₁₎ denotes the intensity of theX-ray diffraction at the (111) plane of the cBN This specificationsuppresses the chipping due to cleavage, enabling the tool to maintainits strength as a cutting tool.

[0017] It is desirable that a base material have a thermal conductivityof 250 to 1,000 W/m·K and a coefficient of thermal expansion of 3.0 to4.0×10⁻⁶/K in a temperature range of 20 to 600° C. Such a high thermalconductivity and a small coefficient of thermal expansion can suppressthe formation of cracks due to heat shock

[0018] It is desirable that the above-described base material have atransverse rupture strength of at least 800 MPa when measured by thethree-point bending method in a temperature range of 20 to 1,000° C. andhave a Vickers hardness of at least 40,000 MPa at room temperature. Thisspecification can improve the mechanical strength of the base materialat high temperatures and decrease chipping during intermittenthigh-speed cutting under the condition of the increased cutting-edgetemperature.

[0019] The above-described base material can be produced by directlyconverting low-crystalline or pulverized high-purity low-pressure BN athigh pressures and high temperatures. Low-pressure BN is a boron nitridethat is thermodynamically stable at normal pressure. The types oflow-pressure BN include hexagonal BN (hBN), rhombohedral BN (rBN),turbostratic BN (tBN), and amorphous BN (aBN). If pBN, which hasintrinsically high orientation, is used as the material, the obtainedcBN sintered body tends to orient in the (111) direction. This highorientation may cause laminar cracks, spalling, or other problems whenthe cBN is used for a cutting tool. When hBN, which has a largecrystal-grain diameter, is used as the material, the rate of conversioninto cBN decreases. Consequently, the production of highly purified cBNrequires increasingly rigorous conditions such as higher pressures andhigher temperatures, which makes it difficult to control thecrystal-grain diameter. On the other hand, when low-crystalline orpulverized high-purity low-pressure BN is used as the starting material,a cBN sintered body having low orientation and a small crystal-graindiameter can be obtained under relatively mild temperature and pressureconditions. In order to reduce the average crystal-grain diameter ofcBN, it is desirable that the sintering temperature be lower than 2,200°C., more desirably between 1,800 and 2,000° C. or so.

[0020] Low-crystalline high-purity low-pressure BN can be obtained byreducing a compound that includes boron and oxygen and using a compoundthat includes carbon and nitrogen. It is desirable that the directconversion from low-pressure BN into cBN be carried out after heatingthe low-pressure BN at a temperature higher than the boiling point ofthe compound that includes boron and oxygen in a non-oxidativeatmosphere.

Hard Coating

[0021] It is desirable that a hard coating be composed of at least onelayer of a compound selected from the group consisting of TiN, TiC,TICN, TIAIN, and Al₂O₃, which have excellent wear resistance in thecutting of steel and cast iron. Of course, a plurality of the samecompound layers can be laminated.

[0022] The present invention features a hard coating having a thicknessof 0.1 to 10 μm. If less than 0.1 μm, the coating cannot increase thewear resistance sufficiently. If more than 10 μm, the coating tends tobe damaged by spalling or chipping.

[0023] It is desirable that a hard coating have a center-line meanroughness of at most 0.1 μm at its surface. This specification canreduce the roughness of the machined surface of a workpiece, improvingthe machining precision. The surface roughness of a hard coating can bereduced by polishing the surface of the base material before coating orby polishing the surface of the hard coating after coating.

[0024] It is desirable that a hard coating be formed on at least theface of a cutting tool. In particular, the coating is the most effectivewhen it is formed in the area from the face to the flank, because thearea plays a principal role in the cutting work. However, the coatingonly on the face also can significantly suppress in particular, thedevelopment of crater wear.

[0025] A hard coating can be formed by the well-known PVD method. Inparticular, the ion-plating method and the sputtering method can form acoating having high quality bonding with the base material. In thiscase, the elastoplastic deformation of the hard coating is restricted atthe interface with the base material while the tool is cutting. As aresult, the hard coating can have further increased hardness and a highbonding strength that precludes spalling.

Intermediate Layer

[0026] It is desirable that an intermediate layer comprising a compoundconsisting mainly of boron and at least one metal element selected fromthe IV a-group elements be provided between the hard coating and thebase material. The presence of the intermediate layer can prolong thetool life because it intensifies the bonding strength between the cBNsintered body and the hard coating. More specifically, it is desirablethat the intermediate layer comprise a boride of a IV a-group element, aboronitride of a IV a-group element, or a mixed composition position ofa boride of a IV a-group element and a nitride of a IV a-group element.Of these groups, it is particularly desirable to use a boride of a IVa-group element, preferably TiB₂.

[0027] It is desirable that an intermediate layer have a thickness of0.05 to 3 μm. A thickness of less than 0.05 μm or more than 3 μm, doesnot improve the bonding quality.

[0028] As with the hard coating, an intermediate layer can be formed bythe PVD method, such as the ion-plating method or the sputtering method.An intermediate layer may be formed solely by the PVD method; it mayalso be formed by the following method: First, a metal layer of a IVa-group element is formed on the base material by the PVD method.Second, the metal layer is heat-treated to react with the cBN in thebase material. This reaction forms a boride of a IV a-group element, aboronitride of a IV a-group element, or a mixed composition of a borideof a IV a-group element and a nitride of a IV a-group element.

Best Mode for Carrying Out the Invention

[0029] The present invention is explained by the following examples.

EXAMPLE 1

[0030] Boron oxide (B₂O₃) and melamine (C₃N₆H₆) were prepared at a moleratio of 3:1 and mixed uniformly with a mortar. The mixed materials weretreated in a tubular furnace in a nitrogen-gas atmosphere at asynthesizing temperature of 850° C. for two hours. The obtained powderwas washed with ethanol to remove unreacted B₂O₃, and treated in ahigh-frequency furnace in a nitrogen-gas atmosphere at 2,100° C for twohours. A gas analysis showed that the obtained boron nitride powder hadan oxygen content of 0.75 wt. %. The oxygen seems to be an impuritypresent as a solid solution in the hBN, because the heat treatment inthe nitrogen gas at 2,100° C. completely removed the B₂O₃ and theadsorbed gases. An X-ray diffraction pattern for the obtained boronnitride showed that no hBN (102) diffraction line was present and thatthe hBN (002) diffraction line was considerably broad, demonstratingsubstantially low crystallinity. The calculation based on the full widthat half maximum of the hBN (002) diffraction line showed that thecrystallite size Lc was 8 nm.

[0031] The obtained low-crystalline low-pressure BN powder wasdie-pressed at 6 ton/cm² to obtain a formed body. The formed body wastreated again in a high-frequency furnace in a nitrogen-gas atmosphereat 2,100° C. for two hours. The treated formed body was placed in amolybdenum capsule to be treated with a belt-typesuperhigh-pressure-generating system at 6.5 GPa and 1,850° C. for 15minutes This treatment produced a sintered body. The obtained sinteredbody contained 99.9 vol. % cBN with an average crystal-grain diameter of0.3 μm. X-ray diffraction showed that the ratio of the cBN (220)diffraction intensity to the cBN (111) diffraction intensity was 0.10.

[0032] The sintered body was machined into the shape of a cutting tip.Its principal surfaces as a tool, namely, the face and flank, werecoated with a layer having a constitution and thickness shown in Table 1by the arc-type ion-plating method. Thus, cutting tools made of thesintered body of the present invention and cutting tools for thecomparative examples were produced. The comparative examples had thefollowing constitutions:

[0033] Comparative example 1-8: Its base material was made of a sinteredbody comprising 99.9 vol. % cBN and had no hard coating.

[0034] Comparative example 1-9: Its base material was made of a sinteredbody comprising 99.9 vol. % cBN and was coated with an extremely thickTiN layer.

[0035] Comparative example 1-10: Its base material was a commerciallyavailable tool for the cutting of hardened steel. The tool was made of asintered body comprising about 60 vol. % cBN and a TiN binder, and wascoated with a TiN layer.

[0036] Comparative example 1-11: Its base material was a commerciallyavailable cemented carbide and was coated with a TiN layer and a TiCNlayer.

[0037] These tools were used for the continuous cutting of an SCM 435round bar (stipulated in the Japanese Industrial Standard; the same isapplied to similar expressions below) having a hardness of H_(S)38 toevaluate their cutting performance. The cutting conditions were asfollows:

[0038] Cutting speed: V=500 m/min.

[0039] Depth of cut: d=0.5 mm

[0040] Feed rate: f=0.15 mm/rev.

[0041] Condition: dry

[0042] The amount of flank wear was measured for evaluation. The resultsare shown in Table 1. TABLE 1 Coating Thickness of coating (μm) (Bottomlayer: at Each layer from Cutting time Width of flank wear Base Materialextreme left) bottom Total (min.) (mm) Present 1-1 99.9% cBN TiN 0.3 0.320 0.3  invention 1-2 99.9% cBN TiN 2.5 2.5 20 0.18 1-3 99.9% cBNTiN/TiAlN 1.0/2.5 3.5 20 0.15 1-4 99.9% cBN TiN/TiCN 0.5/0.5 1.0 20 0.161-5 99.9% cBN TiN/TiC/TiN 0.5/2.0/2.0 4.5 20 0.25 1-6 99.9% cBNTiN/TiCN/TiN 0.5/4.0/4.0 8.5 20 0.14 1-7 99.9% cBN TiC/TiN 2.0/4.0 6.020 0.28 Comparative 1-8 99.9% cBN None — —  5 0.48 example 1-9 99.9% cBNTiN 15.0  15.0   3 Chipping  1-10 Commercial cBN TiN 2.0 2.0 10 0.38 1-11 Commercial cemented TiN/TiCN 0.5/2.0 2.5 10 0.43 carbide

[0043] As can be seen from Table 1, all the examples of the presentinvention were satisfactory in showing the narrow flank wear at acutting time of 20 minutes. On the other hand, Comparative example 1-8,which had no hard coating, resulted in the termination of cutting in anextremely short time, showing wide flank wear. Comparative example 1-9,which had an extremely thick hard coating, developed chipping and alsoresulted in the termination of cutting in an extremely short time.Comparative example 1-10, which contained a small amount of cBN despitebeing provided with a hard coating, showed an unsatisfactory heatresistance of the base material; the flank wear was wide and cutting wasterminated in a short time.

EXAMPLE 2

[0044] As with Example 1, a highly purified low-crystalline low-pressureBN powder was used as the material. A formed body of the material wasplaced in a molybdenum capsule to be treated with a belt-typesuperhigh-pressure-generating system at a pressure of 6.5 GPa and atemperature of 1,850° C. for 15 minutes. This direct conversion produceda cBN sintered body.

[0045] The sintered body was machined into the shape of a cutting tip.Its principal surfaces as a tool, namely, the face and flank, werecoated with a layer having a constitution and thickness shown in Table 2by the arc-type ion-plating method. Thus, cutting tools made of thesintered body of the present invention and cutting tools for thecomparative examples were produced.

[0046] The cutting tool's base material made of the sintered body of thepresent invention contained 99.9 vol. % cBN with an averagecrystal-grain diameter of 0.3 μm. X-ray diffraction showed that theratio of the cBN (220) diffraction intensity to the cBN (111)diffraction intensity was 0.10. The comparative examples were the sameas those used in Example 1.

[0047] These tools were used for the intermittent cutting of an SCM 415round bar having a hardness of H_(B)150, a diameter of 300 mm, and 12V-shaped grooves on the perimeter to evaluate their cutting performance.The cutting conditions were as follows:

[0048] Cutting speed: V=800 m/min.

[0049] Depth of cut: d=0.5 mm

[0050] Feed rate: f=0.15 mm/rev.

[0051] Condition: wet

[0052] The amount of tool wear after a 5-minute cutting and the toollife due to chipping were measured for evaluation. The results are shownin Table 2. TABLE 2 Thickness of Coating coating (μm) Width of flankwear Tool life due to (Bottom layer: at Each layer after 5-min. cuttingchipping Base material extreme left) from bottom Total (mm) (min.)Present 2-1 99.9% cBN TiN 0.3 0.3 0.36 13 invention 2-2 99.9% cBN TiN2.5 2.5 0.32   13.5 2-3 99.9% cBN TiN/TiAlN 1.0/2.5 3.5 0.45 10 2-499.9% cBN TiN/TiCN 0.5/0.5 1.0 0.28 18 2-5 99.9% cBN TiN/TiC/TiN0.5/2.0/2.0 4.5 0.24   12.5 2-6 99.9% cBN TiN/TiCN/TiN 0.5/4.0/4.0 8.50.30 15 2-7 99.9% cBN TiC/TiN 2.0/4.0 6.0 0.48 13 Com- 2-8 99.9% cBNNone — — 0.72   5.8 parative 2-9 99.9% cBN TiN 5.0 15.0  Chipping  3example  2-10 Commercial TiN 2.0 2.0 Chipping due to   0.8 cBN thermalcracks  2-11 Commercial TiN/TiCN 0.5/2.0 2.5 Chipping   0.3 cementedcarbide

[0053] As can be seen from Table 2, all the examples of the presentinvention also had a long tool life determined by the development ofchipping in intermittent cutting. On the other hand, all the comparativeexamples had a short tool life due to chipping, showing inferiorresistance to wear and chipping

EXAMPLE 3

[0054] As with Example 1, a highly purified low-crystalline low-pressureBN powder was used as the material. A formed body of the material wasplaced in a molybdenum capsule to be treated with a belt-typesuperhigh-pressure-generating system at a pressure of 6.5 GPa and atemperature of 1,800 to 2,000° C. for 15 minutes. This treatmentproduced a cBN sintered body. Table 4 shows the composition of theobtained cBN sintered body, the crystal-grain diameter of the cBN, andthe ratio of the cBN (220) diffraction intensity to the cBN (111)diffraction intensity in X-ray diffraction.

[0055] The cBN sintered body was machined into the shape of a cuttingtip Its face and flank were coated with a TiN layer having a thicknessof about 1.5 μm by the ion-plating method. The following comparativeexamples were also prepared:

[0056] Comparative example 3-6: Its cBN sintered body was obtained bytreating the material at 6.5 GPa and an increased temperature of 2,200°C. for 15 minutes. The obtained cBN sintered body was machined into theshape of a cutting tip. Its was coated with a TiN layer having athickness of about 1.5 μm by the same method as mentioned above tocomplete the tool.

[0057] Comparative example 3-7: Its cBN sintered body was obtained bytreating the material, in this case pBN, at 6.5 GPa and 1,850° C. for 15minutes. The obtained cBN sintered body was machined into the shape of acutting tip. It was coated with a TiN layer having a thickness of about1.5 μm by the same method as mentioned above to complete the tool.

[0058] Comparative example 3-8: Its base material was a commerciallyavailable tool for the cutting of hardened steel. The tool, made of asintered body comprising about 60 vol. % cBN and a TiN binder, wascoated with a TiN layer by the same method as mentioned above

[0059] A cutting test was carried out by continuously cutting an SCM 415round bar having a hardness of H_(B)180 under the following conditions:

[0060] Cutting speed: V=500 m/min.

[0061] Depth of cut: d=0.5 mm

[0062] Feed rate: f=0. 15 mm/rev.

[0063] Condition: wet

[0064] The amount of flank wear of the tool was measured after a10-minute cutting. The results are also shown in Table 3. TABLE 3 Ratioof X-ray Sintering Crystal-grain diffraction in- Amount of temperaturediameter CBN content tensities of cBN Cutting time flank wear (° C.)(μm) (vol. %) I₍₂₂₀₎/I₍₁₁₁₎ (min.) (mm) Present 3-1 1,800 At most 0.599.7 0.22 10 0.113 invention 3-2 1,850 At most 0.5 99.9 0.10 10 0.1083-3 1,880 At most 0.5 99.9 0.12 10 0.120 3-4 1,950 0.5-1 99.9 0.08 100.096 3-5 2,000 0.5-1 100   0.08 10 0.110 Comparative 3-6 2,200   3-5100   0.18   1.2 Chipping example 3-7 (Material: 1,850 At most 0.5 99.80.04   1.6 Chipping pBN) 3-8 (Commercial — 0.5-4 60   —  1 Chipping duetool) to thermal cracks

[0065] As can be seen from the data on the examples of the presentinvention in Table 3, it is desirable that the base material be sinteredat 1,800 to 2,000° C. or so. On the other hand, all the comparativeexamples developed chipping, showing a short tool life.

[0066] In addition, the base materials for Example 3-1 to 3-5 of thepresent invention and Comparative example 3-6 to 3-8 were subjected tothe measurements of hardness, transverse rupture strength, a coefficientof thermal expansion, and thermal conductivity. The hardness wasmeasured by the Vickers hardness. The transverse rupture strength wasmeasured by the three-point bending method by varying the atmospherictemperature from 20 to 1,000° C. The test specimen had a size of 6×3×0.7mm (span: 4 mm). The coefficient of thermal expansion was measured byvarying the temperature from 20 to 600° C. The measured results areshown in Table 4. TABLE 4 H_(V) hardness Transverse rupture strengthCoefficient of thermal Thermal (MPa) (MPa) expansion (×10⁻⁶/K)conductivity 20° C. 20° C. 500° C. 1,000° C. 20° C. 300° C. 600° C. (W/m· K) Example 3-1 44,100 1,010 1,060 1,280 3.6 3.8 3.9 360 3-2 49,5001,250 1,320 1,500 3.6 3.8 3.9 420 3-3 51,000 1,320 1,280 1,620 3.6 3.83.9 420 3-4 57,400 1,240 1,180 1,030 3.3 3.5 3.7 470 3-5 56,900 1,0201,130   940 3.3 3.5 3.6 510 Comparative 3-6 54,900   610   590   370 3.13.3 3.6 580 example 3-7 52,200 1,030   720   590 3.5 3.8 3.9 385 3-830,900 1,130   932   412 4.2 4.5 4.9  60

[0067] As can be seen from Tables 3 and 4, the following samples showedsatisfactory results:

[0068] {circle over (1)}: the sample whose base material has a Vickershardness of at least 40,000 MPa at room temperature

[0069] {circle over (2)}: the sample whose base material has atransverse rupture strength of at least 800 MPa when measured by thethree-point bending method at a temperature range of 20 to 1,000° C.

[0070] {circle over (3)}: the sample whose base material has acoefficient of thermal expansion of 3.0 to 4.0×10⁻⁶/K at a temperaturerange of 20 to 600° C.

[0071] {circle over (4)}: the sample whose base material has a thermalconductivity of 250 to 1,000 W/m·K.

EXAMPLE 4

[0072] As with Example 1, a highly purified low-crystalline low-pressureBN powder was used as the material. A formed body of the material wasplaced in a molybdenum capsule to be treated with a belt-typesuperhigh-pressure-generating system at a pressure of 6.5 GPa and atemperature of 1,850° C. for 15 minutes. This direct conversion produceda cBN sintered body. The cBN sintered body was machined into the shapeof a cutting tip having a nose radius of 0.2 mm and a relief angle of 7degrees at the tip end. The cBN sintered body's surface corresponding tothe face was mirror-polished, and the surface corresponding to the flankwas ground by using a No. 3,000 grinding wheel such that the remainingchipping at the tip end became at most 1 μm in width.

[0073] The cBN sintered body's principal surfaces as a tool, namely, theface and flank, were coated with a TiN layer having a thickness shown inTable 4 and a center-line mean roughness of at most 0 μm at its surfaceby the arc-type ion-plating method. Thus, cutting tools made of thesintered body of the present invention were produced. The comparativeexamples were produced such that their coated surface had a center-linemean roughness exceeding 0.1 μm.

[0074] These tools were used for the continuous cutting of a round barmade of an SUJ2 bearing steel having a hardness of H_(RC) 56 and adiameter of 80 mm. The cutting conditions were as follows:

[0075] Cutting speed: V=30 m/min.

[0076] Depth of cut: d=0.05 mm

[0077] Feed rate: f=0. 01 mm/rev.

[0078] Condition: dry

[0079] The surface roughness “Rmax” of the machined workpieces wasmeasured after a 3-minute cutting. The results are shown in Table 5.TABLE 5 Thickness of Surface rough- Surface roughness of coating ness ofcoating machined workpiece (μm) (μm) Rmax (μm) Present 4-1 0.5 0.03 0.08invention 4-2 0.5 0.07 0.18 4-3 2.5 0.05 0.17 4-4 3.0 0.04 0.10 4-5 6.00.08 0.21 Comparative 4-6 0.5 0.23 0.53 example 4-7 15   0.85 1.82

[0080] As can be seen from Table 5, the examples of the presentinvention, which had a center-line mean roughness of at most 0.1 μm atthe surface of their hard coating, showed small surface roughness in themachined workpieces, demonstrating high-precision cutting. On the otherhand, the comparative examples, which had a large center-line meanroughness at the surface of their hard coating, showed large surfaceroughness in the machined workpieces. In particular, Comparative example4-7, which had an excessively thick hard coating, developed spallingaway of the hard coating, showing large surface roughness in themachined workpieces.

EXAMPLE 5

[0081] A low-crystalline low-pressure BN powder was die-pressed at 6ton/cm² to obtain a formed body. The formed body was highly purified ina high-frequency furnace in a nitrogen-gas atmosphere at 2,100° C. fortwo hours. The treated formed body was placed in a molybdenum capsule tobe treated with a belt-type super high-pressure generating system at 6.5GPa and 1,850° C. for 15 minutes. This treatment produced a sinteredbody containing 99.9 vol % cBN with an average crystal-grain diameter of0.3 μm. X-ray diffraction showed that the ratio of the cBN (220)diffraction intensity to the cBN (111) diffraction intensity was 0.10.

[0082] The sintered body was machined into the shape of a cutting tip.The surfaces involved in the cutting work as a tool, namely, the faceand flank, were coated with an intermediate layer. Next, theintermediate layer was covered with a hard coating having a constitutionand thickness shown in Table 1 by using the arc-type ion-plating method.An oxide layer constituting a part of the hard coatings was formed bythe magnetron sputtering method. Thus, the tools for the examples of thepresent invention, those for the reference examples, and those for thecomparative examples were produced.

[0083] The intermediate layer was formed by either of the following twomethods:

[0084] (a) First, a layer of a IV a-group metal was formed by thearc-type ion-plating method. Second, the metal layer was heat-treated at1,100° C. for 30 minutes in a vacuum at a pressure of 1.3×10⁻³ Pa toreact with the cBN. This reaction produced a layer made of a boride of aIV a-group element or made of a mixed composition of a boride of a IVa-group element and a nitride of a IV a-group element.

[0085] (b) A layer similar to that in (a) was formed only by thearc-type ion-plating method. In Table 6, the method (a) is referred toas “heat treatment” and the method (b) is referred to as “PVD only.”

[0086] The reference examples were prepared as follows:

[0087] Reference example 5-13: No intermediate layer was provided.

[0088] Reference example 5-14: The intermediate layer was significantlythick.

[0089] Reference example 5-15: The base material had a cBN content of99.5%

[0090] The types of comparative examples included an example that wasproduced by coating a commercially available cBN-sintered-body tool witha hard coating and an example that was a commercially available coatedcemented-carbide tool. Comparative examples were prepared as follows:

[0091] Comparative example 5-18: A commercially available tool made of acBN sintered body for the cutting of hardened steel was coated withTiAIN. The cBN sintered body contained TiN as a binder and its cBNcontent was about 60 vol. %

[0092] Comparative example 5-19: This example was a commerciallyavailable cemented-carbide tool coated with TiAIN.

[0093] These tools were used for the continuous cutting of an SCM 435round bar having a hardness of H_(S) 35 to evaluate their cuttingperformance. The cutting conditions were as follows:

[0094] Cutting speed: V=800 m/min.

[0095] Depth of cut: d=0.5 mm

[0096] Feed rate: f=0.15 mm/rev.

[0097] Condition: dry

[0098] The cutting was carried out for at most 15 minutes to measure theamount of flank wear of the tool. The results are shown in Table 6.TABLE 6 Thickness of Coating Thickness of coating Width CBN contentintermediate (Bottom (each layer from Cutting of flank of baseIntermediate layer Production layer: bottom) time wear material layer(μm) method at left) (μm) (min.) (mm) Example 5-1  99.9% TiB₂  0.07 Heattreatment TiN  0.35 15 0.43 5-2  99.9% TiB₂ 0.3 Heat treatment TiCN 1.015 0.27 5-3  99.9% TiB₂ 0.3 Heat treatment TiN/TiAlN 0.5/2.0 15 0.225-4  99.9% TiB₂ 0.2 Heat treatment Al₂O₃/TiN 4.5/1.5 15 0.15 5-5  99.9%TiB₂ 2.5 Heat treatment TiAlN 4.0 15 0.40 5-6  99.9% TiB₂, TiN 1.0 Heattreatment TiCN 2.0 15 0.25 5-7  99.9% TiB₂ 0.1 Heat treatment ZrN 2.0 150.16 5-8  99.9% TiB₂ 0.3 PVD only (Ti, Zr) N 6.5 15 0.20 5-9  99.9% TiB₂1.0 PVD only TiCN 1.5 15 0.33 5-10 99.9% TiBN 0.5 PVD only TiAlN 1.0 150.32 5-11 99.9% TiB₂, TiN 0.1 Heat treatment TiAlN 0.3 15 0.42 5-1299.9% TiBN 0.5 PVD only TiCN/TiN 6.5/2.0 15 0.30 Reference 5-13 99.9%None TiAlN 3.0  2 0.76 example 5-14 99.9% TiB₂ 5.0 Heat treatment TiCN2.0  5 0.92 5-15 99.5% TiB₂ 0.3 PVD only TiCN 2.5 12 ChippingComparative 5-16 99.9% TiB₂ 0.3 Heat treatment TiCN 13.0   2 0.68example 5-17 98.8% TiB₂, TiN 0.2 Heat treatment TiCN 2.5 10 Chipping5-18 Commercial TiB₂ 0.3 PVD only TiAlN 3.0  1 0.52 cBN 5-19 CommercialTiB₂ 0.5 PVD only TiAlN 3.0   0.2 Chipping cemented carbide

[0099] As can be seen from Table 6, all the examples of the presentinvention, which were provided with an intermediate layer, were capableof 15-minute cutting, showing small amounts of wear. On the other hand,the reference example having an intermediate layer as thick as 5 μmresulted in the termination of cutting in a short time.

EXAMPLE 6

[0100] The tools provided in Example 5 as the examples, the referenceexamples, and the comparative examples were used for the intermittentcutting of an SCM 415 round bar having a hardness of H_(B) 150 and adiameter of 300 mm. The round bar was provided with 12 V-shaped grooveson its perimeter to apply heat shock. The cutting conditions were asfollows:

[0101] Cutting speed: V=800 m/min.

[0102] Depth of cut: d=0.5 mm

[0103] Feed rate: f=0.15 mm/rev.

[0104] Condition: wet

[0105] The cutting was carried out to measure the tool life due tochipping. The results are shown in Table 7. TABLE 7 Thickness ofThickness of coating Tool life intermediate Coating (each layer due toCBN content of Intermedi- layer Production (Bottom from bottom) chippingbase material ate layer (μm) method layer: at left) (μm) (min.) Example6-1  99.9% TiB₂  0.07 Heat treatment TiN  0.35 11 6-2  99.9% TiB₂ 0.3Heat treatment TiCN 1.0 15 6-3  99.9% TiB₂ 0.3 Heat treatment TiN/TiAlN0.5/2.0 13 6-4  99.9% TiB₂ 0.2 Heat treatment Al₂O₃/TiN 4.5/1.5 10 6-5 99.9% TiB₂ 2.5 Heat treatment TiAlN 4.0  7 6-6  99.9% TiB₂, TiN 1.0 Heattreatment TiCN 2.0   13.5 6-7  99.9% TiB₂ 0.1 Heat treatment ZrN 2.0 156-8  99.9% TiB₂ 0.3 PVD only (Ti, Zr) N 6.5   8.5 6-9  99.9% TiB₂ 1.0PVD only TiCN 1.5 12 6-10 99.9% TiBN 0.5 PVD only TiAlN 1.0 11 6-1199.9% TiB₂, TiN 0.1 Heat treatment TiAlN 0.3 12 6-12 99.9% TiBN 0.5 PVDonly TiCN/TiN 6.5/2.0   7.5 Reference 6-13 99.9% None TiAlN 3.0   3.5example 6-14 99.9% TiB₂ 5.0 Heat treatment TiCN 2.0  3 6-15 99.5% TiB₂0.3 PVD only TiCN 2.5  3 Comparative 6-16 99.9% TiB₂ 0.3 Heat treatmentTiCN 13.0    1.6 example 6-17 98.8% TiB₂, TiN 0.2 Heat treatment TiCN2.5   1.2 6-18 Commercial TiB₂ 0.3 PVD only TiAlN 3.0   0.1 cBNCommercial TiB₂ 0.5 PVD only TiAlN 3.0    0.05 6-19 cemented car- bide

[0106] As can be seen from Table 7, the examples of the presentinvention, which were provided with an intermediate layer, showed a longtool life determined by the development of chipping even in intermittentcutting.

EXAMPLE 7

[0107] As with Example 5, a highly purified low-crystalline low-pressureBN powder was used as the material. A formed body of the material wasplaced in a molybdenum capsule to be treated with a belt-typesuperhigh-pressure-generating system at a pressure of 6.5 GPa and atemperature of 1,800 to 2,000° C. for 15 minutes. This treatmentproduced a cBN sintered body. Table 8 shows the composition of theobtained cBN sintered body, the crystal-grain diameter of the cBN, theratio of the cBN (220) diffraction intensity to the cBN (111)diffraction intensity in X-ray diffraction.

[0108] The cBN sintered body was machined into the shape of a cuttingtip. Its face and flank were coated with a Ti layer having a thicknessof about 0.5 μm by the ion-plating method. The Ti-coated sintered bodywas heat-treated in a vacuum furnace at 1,100° C. for 30 minutes in avacuum at a pressure of 1.3×10⁻³ Pa to form an intermediate layer madeof TiB₂. The TiB₂ layer was then coated with a TiAIN layer having athickness of about 1.0 μm. Thus, the examples of the present inventionwere produced. The following tools were also prepared as comparativeexamples to be subjected to an evaluation test of cutting performance:

[0109] Comparative example 7-6: Its cBN sintered body was obtained bytreating the material at 6.5 GPa and an increased temperature of 2,200°C. for 15 minutes. The obtained cBN sintered body was coated with a TiNlayer having a thickness of about 1.5 μm to complete the tool.

[0110] Comparative example 7-7: Its cBN sintered body was obtained bytreating the material, in this case pBN, at 6.5 GPa and 1,850° C. for 15minutes. The obtained cBN sintered body was coated with a TiN layerhaving a thickness of about 1.5 μm to complete the tool.

[0111] Comparative example 7-8: The base material was a commerciallyavailable cBN-sintered-body tool for the cutting of hardened steel. Thisbase material contained TiN as a binder, and its cBN content was about65 vol. %. The base material was coated with TiB2 and TiAIN by theion-plating method.

[0112] A cutting test was carried out by intermittently cutting an SCM415 round bar having a hardness of H_(B) 180 and a diameter of 300 mm.The round bar was provided with 12 V-shaped grooves on its perimeter toapply heat shock. The cutting conditions were as follows:

[0113] Cutting speed: V=500 m/min.

[0114] Depth of cut: d=0.5 mm

[0115] Feed rate: f=0.15 mm/rev.

[0116] Condition: wet

[0117] The amount of flank wear of the tool was measured after a10-minute cutting. The results are also shown in Table 8. The basematerials were subjected to the measurements of hardness, transverserupture strength from room temperature to 1,000° C., a coefficient ofthermal expansion from room temperature to 600° C., and thermalconductivity by using methods similar to those used for obtaining theresults in Table 4 described in Example 3. The measured results weresimilar to those shown in Table 4. TABLE 8 Ratio of X-ray SinteringCrystal-grain diffraction in- Amount of temperature diameter CBN contenttensities of cBN Cutting time flank wear (° C.) (μm) (vol. %)I₍₂₂₀₎/I₍₁₁₁₎ (min.) (mm) Example 7-1 1,800 At most 0.5 99.7 0.22 100.086 7-2 1,850 At most 0.5 99.9 0.10 10 0.094 7-3 1,880 At most 0.599.9 0.12 10 0.081 7-4 1,950 0.5-1 99.9 0.08 10 0.102 7-5 2,000 0.5-1100   0.08 10 0.110 Com- 7-6 2,200   3-5 100   0.18   0.1 Chippingparative 7-7 (Material: 1,850 At most 0.5 99.8 0.04   1.0 Chippingexample pBN) 7-8 (Commer- — 0.5-4 60   —   0.2 Chipping due to cialtool) thermal cracks

[0118] As can be seen from the data on the examples of the presentinvention in Table 8, it is desirable that the base material be sinteredat 1,800 to 2,000° C. or so. On the other hand, all the comparativeexamples developed chipping, showing a short tool life.

[0119] Table 8 in collaboration with Table 4 reveals that the followingsamples showed satisfactory results:

[0120] {circle over (1)}: the sample whose base material has a Vickershardness of at least 40,000 MPa at room temperature

[0121] {circle over (2)}: the sample whose base material has atransverse rupture strength of at least 800 MPa when measured by thethree-point bending method at a temperature range of 20 to 1,000° C.

[0122] {circle over (3)}: the sample whose base material has acoefficient of thermal expansion of 3.0 to 4.0×10⁻⁶/K at a temperaturerange of 20 to 600° C.

[0123] {circle over (4)}: the sample whose base material has a thermalconductivity of 250 to 1,000 W/m·K.

EXAMPLE 8

[0124] As with Example 5, a highly purified low-crystalline low-pressureBN powder was used as the material. A formed body of the material wasplaced in a molybdenum capsule to be treated with a belt-type superhigh-pressure generating system at a pressure of 6.5 GPa and atemperature of 1,850° C. for 15 minutes. This direct conversion produceda cBN sintered body. The cBN sintered body was machined into the shapeof a cutting tip having a nose radius of 0.4 mm and a relief angle of 7degrees at the tip end. The cBN sintered body's surface corresponding tothe face was mirror-polished by using a diamond abrasive grain having adiameter of at most 3 μm. The surface corresponding to the flank wasground by using a No. 3,000 grinding wheel such that the remainingchipping at the tip end became at most 1 μm in width. The cBN sinteredbody's principal surfaces as a tool, namely, the face and flank, werecoated with a Ti layer having a thickness of about 0.08 μm by theion-plating method. The Ti-coated sintered body was heat-treated in avacuum furnace at 1,100° C. for 30 minutes in a vacuum at a pressure of1.3×10⁻³ Pa to form an intermediate layer made of TiB₂. The TiB₂ layerwas then coated with a TiAIN layer having a thickness shown in Table 9and a center-line mean roughness of at most 0.1 μm at its surface by theion-plating method. Thus, tools were produced as the examples of thepresent invention. The comparative examples were produced such thattheir coated surface had a center-line mean roughness exceeding 0.1 μm.

[0125] These tools were used for the continuous cutting of a round barmade of SUJ2 bearing steel having a hardness of H^(RC) 60 and a diameterof 20 mm. The cutting conditions were as follows:

[0126] Cutting speed: V=100 m/min.

[0127] Depth of cut: d=0.05 mm

[0128] Feed rate: f=0.03 mm/rev.

[0129] Condition: dry

[0130] The surface roughness of the machined workpieces was measuredafter 3-minute cutting. The results are shown in Table 9. TABLE 9Thickness of Surface rough- Surface roughness of TiAlN layer ness ofcoating machined workpiece (μm) (μm) Rmax (μm) Example 8-1 0.3 0.03 0.58-2 0.5 0.06 0.6 8-3 1.0 0.05 0.5 8-4 3.0 0.08 0.6 8-5 5.0 0.09 0.7Comparative 8-6 0.5 0.23 1.0 example 8-7 15   0.85 1.6

[0131] As can be seen from Table 9, the examples of the presentinvention, which had a center-line mean roughness of at most 0.1 μm atthe surface of their hard coating, showed small surface roughness in themachined workpieces, demonstrating high-precision cutting. On the otherhand, the comparative examples, which had a large center-line meanroughness at the surface of their hard coating, showed large surfaceroughness in the machined workpieces. In particular, Comparative example8-7, which had an excessively thick hard coating, developed spallingaway of the hard coating, showing large surface roughness in themachined workpieces.

[0132] As a matter of course, the coated cBN sintered body of thepresent invention shall not be limited to the above-describedembodiments, and it can be modified within the scope that does notdeviate from the essential points of the present invention.

Industrial Applicability

[0133] As explained above, the coated cBN sintered body of the presentinvention enables the realization of high resistance to wear and heat bycoating a base material consisting essentially of cBN with a hardcoating comprising at least one compound layer consisting mainly of (a)at least one metal element selected from the group consisting of Al andthe IV a-group elements and (b) at least one element selected from thegroup consisting of C, N, and O. In particular, these effects can bemanifested notably in the high-speed cutting of steel.

[0134] The present invention also provides between the base material andthe hard coating an intermediate layer comprising a compound consistingmainly of boron and at least one metal element selected from the IVa-group elements. The presence of the intermediate layer can prolong thetool life because it intensifies the bonding strength between the basematerial and the hard coating.

1. A coated cubic boron nitride sintered body comprising: (a) a basematerial made of a sintered body comprising at least 99.5 vol. % cubicboron nitride (cBN); and (b) a hard coating that: (b1) is formed on atleast part of the surface of the base material by the physical vapordeposition method; and (b2) has a thickness of 0.1 to 10 μm.
 2. A coatedcBN sintered body as defined in claim 1, wherein the hard coatingcomprises at least one compound layer consisting mainly of: (a) at leastone metal element selected from the group consisting of Al and the IVa-group elements; and (b) at least one element selected from the groupconsisting of C, N, and O.
 3. A coated cBN sintered body as defined inclaim 1, the sintered body further comprising an intermediate layerthat: (a) is provided between the hard coating and the base material;and (b) comprises a compound consisting mainly of boron and at least onemetal element selected from the IV a-group elements.
 4. A coated cBNsintered body as defined in claim 3, wherein the intermediate layercomprises a boride of a IV a-group element.
 5. A coated cBN sinteredbody as defined in claim 3, wherein the intermediate layer comprisesTiB₂.
 6. A coated cBN sintered body as defined in claim 3, wherein theintermediate layer has a thickness of 0.05 to 3 μm.
 7. A coated cBNsintered body as defined in claim 1, wherein the cBN constituting thebase material comprises crystal grains having an average crystal-graindiameter of at most 1 μm.
 8. A coated cBN sintered body as defined inclaim 1, wherein the ratio I₍₂₂₀₎/I₍₁₁₁₎ is at least 0.05 in the X-raydiffraction lines in an arbitrary direction of the base material, whereI₍₂₂₀₎ denotes the (220) diffraction intensity, and I₍₁₁₁₎ denotes the(111) diffraction intensity.
 9. A coated cBN sintered body as defined inclaim 1, wherein the base material comprises at least 99.9 vol. % cBN.10. A coated cBN sintered body as defined in claim 1, wherein the basematerial has a thermal conductivity of 250 to 1,000 W/m·K.
 11. A coatedcBN sintered body as defined in claim 1, wherein the base material has acoefficient of thermal expansion of 3.0 to 4.0×10⁻⁶/K in a temperaturerange of 20 to 600° C.
 12. A coated cBN sintered body as defined inclaim 1, wherein the base material has a transverse rupture strength ofat least 800 MPa when measured by the three-point bending method in atemperature range of 20 to 1,000° C.
 13. A coated cBN sintered body asdefined in claim 1, wherein the base material has a Vickers hardness ofat least 40,000 MPa at room temperature.
 14. A coated cBN sintered bodyas defined in claim 1, wherein the hard coating has a center-line meanroughness of at most 0.1 μm at its surface.